Watering from the eyes due to narrowing or occlusion of the tear ducts, that is to say the tear drainage ducts, is a common problem. In a healthy individual the tear drainage system collects the tears from the inner corner of the eye through a small opening (punctum) in the margin of the eyelid, there being one punctum in each of the upper and lower eyelids. Each punctum leads to a canaliculus which passes horizontally through the medial end of the eyelid towards the nose, the canaliculi usually joining to form a single common canaliculus as they reach the lacrimal sac. Here the tear duct changes to an inferior direction passing downward to become the lacrimal duct and finally exiting into the lower part of the nose.
Narrowing or occlusion of the tear duct can occur at any point in its course from the eye to nose. Typically, the evolution of tear duct obstruction involves a progressive narrowing of the tear duct from an initially fully open state, through in some cases to complete occlusion. The consequent reduction in tear drainage leads to troublesome watering from the eye, soreness of the eyelids, and sometimes infections.
Well-established techniques of tear duct surgery are available to improve drainage. Surgery is usually of value where complete obstruction exists and is often indicated before the system is completely obstructed as troublesome watering can still be corrected. It is known that many patients with eye watering do not have completely occluded tear systems. In these cases it can often be difficult to ensure that the tear duct is the cause of watering from the eye, to monitor the process of narrowing, to decide when to intervene, and to assess the response to treatment. In general, the greater the degree of narrowing the higher is the likelihood of a successful outcome from surgery. A test that could accurately measure the degree of narrowing in a simple and safe way would be very useful.
Several clinical tests can be used to help decide how narrow the tear duct is. For example, basic information can be derived by examining the tear film height and estimating the speed of clearance of a drop of fluorescein colouring in the tear film. Jones tests, which rely on identifying passage of fluorescein to the nose, have been advocated for assessing watering where the tear system is at least partly open, but are known to have high levels of inaccuracy. Radiological tests looking at the anatomy and physiological function of the tear system are also known, but can be expensive and time-consuming, and are subjective and prone to errors of administration or interpretation.
In practice the mainstay of clinical examination is to use a lacrimal cannula inserted into the punctum and connected to a syringe to irrigate fluid down the tear system. The syringe and cannula are hand-held, fluid is irrigated under pressure and the passage of fluid to the nose or regurgitation back from the same or, because they are connected, the opposite punctum, is identified. With experience a subjective estimate can be made of the level of resistance to fluid flow.
Tucker et al (Ophthalmology, Vol. 102, No. 11 (November 1995) p. 1639) has described a more objective measure of lacrimal resistance, where resistance=pressure/flow. By sealing an irrigating cannula tip at the punctum, irrigating with water at a known flow rate, and recording the pressure generated, figures for resistance were derived in normal subjects and those with open tear ducts following successful lacrimal surgery. However the research equipment used has a number of drawbacks which would prevent application in a clinical environment and use in those where tear duct narrowing or occlusion is present, as is usually the case.